Subnetting 101

IPv4 Overview

This is intented to give a brief review on IPv4 and discover the magic of Routing IPv4 packets then unravel the secrets of Subnetting.

Basically, the Internet Protocol Address Version 4(IPv4) has 3 different usable classes, namely:Class A, Class B and Class C. There is also a group of addresses used for multicast called Class D and another group of reserved addresses used for research called, Class E. This document will focus only on Class A, B, and C.

IPV4 Review

In order to start unraveling the mystery of subnetting, let us try first to review the characteristics of an IPv4 address. An IPv4 address as we all know is 32-bits in size which is divided into 4 groups called octets. Each octet represents 8-bits of data(4 groups x 8-bits per group equals 32, right?). In writing this 32-bit IP address, each octet is separated with a dot(.) and the leftmost octet is the most significant octet and the rightmost octet is the least significant octet.

IPv4 Address Binary Representation:

In order to be able to identify to which class does an IPv4 address belong, we look at its most significant bits(the leftmost bits) of the most signifacant octet of an IPv4 address.


In Figure 2, if you try to examine a Class A address, you will find that its most significant bit is equal to 0. The first two most significant of a Class B address is 10 and Class C address starts with 110. The x's found in Figure 2 represents either 0 or 1.

Simplifying the IP address:

Suppose we have an IP address in binary: 11000011101010100101010100001111 (32 bits)
It will be divided into 4 octets separated with a dot: 11000011.10101010.01010101.00001111

But binary numbers are hard to memorize, so an IPv4 address is commonly represented in decimal formats.
11000011.10101010.01010101.00001111 is now represented as 195.170.85.15

Binary to decimal conversion:
11000011 = 195
10101010 = 170
01010101 = 85
00001111 = 15

CCNA Practice Problems:

Q1: The IPv4 address 195.170.85.15 belongs to what class?
Q2: What could be the minimum and maximum value for the first octet of a Class A address?
Q3: What could be the minimum and maximum value for the first octet of a Class B address?
Q4: What could be the minimum and maximum value for the first octet of a Class C address?

Answers will be posted in the next blog related to this article.


To be continued...
Next topic: The Magic of Routing